1. Field of the Invention
The present invention relates to an excavation tool for use in the excavation of earth and sand, such as the digging of wells or the construction of foundation piling holes and the like.
2. Prior Art
Conventionally, a type of excavation toll for the excavation of earth and sand was known in which a pilot bit was provided at the lead end of an excavation pipe, by means of this pilot bit the bottom surface of an excavation hole was excavated and the diameter of the excavation hole was widened by means of an eccentric reamer disposed at the upper part of this pilot bit, and using the widened part of the excavation hole, the excavation pipe was advanced. However, in this type of excavation tool, a part of the circumference of the excavation hole was re-excavated by means of the eccentric reamer, and an eccentric hole thus created, so that there was a disadvantage in that the excavation hole bent easily.
Recently, excavation tools such as that disclosed in Japanese Patent Application, first publication, Laid open No. Sho. 63-11789 have been developed in order to solve this problem.
As shown in FIGS. 1 through 3, this excavation tool is provided with a device 2 which receives the striking force of a hammer (not depicted in the diagram) and the rotational force of a hammer ring 1; two axle holes 2a and 2b are formed in the bottom surface thereof which are in point symmetry with respect to the center of device 2, and block axles 3a and 3b are inserted into these axle holes 2a and 2b in a freely rotatable and firmly attached manner. At the lead end parts of these block axles 3a and 3b, blocks 5a and 5b, which have roughly the same diameter as that of the above device 2, are roughly semicircular in shape, and have embedded in lead end surfaces thereof a plurality of bits 4..., are provided with the straight edge surfaces 6a and 6b thereof in mutual opposition. When the above device 2 rotates in the direction of excavation, the positions of the above block axles 3a and 3b are such that one end of each of the above blocks 5a and 5b protrudes beyond the outer circumferential surface of device 2 by a predetermined excavation amount, and at this time, the straight edge surfaces 6a and 6b of the blocks are displaced from the center of device 2 so that they are in mutual contact.
When device 2 is rotated in the direction of excavation X by means of hammer ring 1, in the above excavation tool, blocks 5a and 5b rotate about block axles 3a and 3b while receiving excavation resistance, one end of the straight edge surfaces 6a and 6b of blocks 5a and 5b protrudes beyond the outer circumferential surface of device 2 by a prescribed amount, a part of straight edge surfaces 6a and 6b come into mutual contact and stop the rotation of blocks 5a and 5b, and in this state, blocks 5a and 5b receive the rotational force of device 2, earth is excavated by means of bits 4..., and advancement is made into the earth by means of the striking force of the hammer.
At this time, the earth and sand and the like which is excavated is separated from the lead end of the excavation tool by means of the blowing of compressed air, expelled at the time of the falling of the piston hammer within hammer cylinder 1, from airholes 8a and 8b provided in the bottom surface of device 2, and after this, the earth and sand and the like is moved to the interior of excavation pipe 9 through exhaust grooves 9a and is finally expelled in an upwards direction.
In the above excavation tool, as shown in FIG. 2, earth is excavated by means of a part of blocks 5a and 5b which protrudes in an outward direction beyond the outer circumferential surface of device 2 (hereinafter termed the outer circumferential blade A); these outer circumferential blades A are only present at two points separated by 180 degrees and beyond the outer circumferential surface of device 2. This excavation tool is superior to the eccentric hole excavating type discussed above; however, it is incapable of balanced excavation, so that for example in the case in which layers of uneven quality are excavated, there is a danger that hole bending will be produced.
Furthermore, as shown in FIG. 2, when an excavation counterforce C which is parallel to straight edge surfaces 6a and 6b is placed on blocks 5a and 5b from the inner circumferential surface of the excavation hole, the friction between straight edge surfaces 6a and 6b is negligibly small, so that almost all the excavation counterforce C acts on block axle 3a(3b) through the medium of the blocks, and a great load is placed on block axle 3a(3b).
Furthermore, an appropriate amount of play is provided between axle holes 2a and 2b and block axles 3a and 3b so that the block axles are able to rotate; however, when the above-described excavation counterforce C is placed on blocks 5a and 5b, rattling is produced in blocks 5a and 5b along straight edge surfaces 6a and 6b in accordance with the extent of the above play, and since it is impossible to firmly fasten the blocks in this manner, the lifespan of the tool is shortened, and excavation efficiency declines.